Thin film semiconductor device and manufacturing method therefor

ABSTRACT

A thin film semiconductor device comprises a substrate, a gate electrode disposed above the substrate, an oxide semiconductor layer disposed above the substrate so as to oppose the gate electrode, a channel protective layer disposed on the oxide semiconductor layer, and a source electrode and a drain electrode each connected to the oxide semiconductor layer. The density of states DOS [eV −1 cm −3 ] of oxygen defects in the oxide semiconductor layer satisfies the following relationship:
 
DOS≦1.710×10 17 ×( Ec−E ) 2 −6.468×10 17 ×( Ec−E )+6.113×10 17  
 
provided that 2.0 eV≦Ec−E≦2.7 eV
 
where Ec [eV] is an energy level of a conduction band edge of the oxide semiconductor layer and E [eV] is a predetermined energy level of the oxide semiconductor layer.

BACKGROUND

1. Field

This disclosure relates to a thin film semiconductor device and a methodfor manufacturing the thin film semiconductor device.

2. Description of the Related Art

Thin film transistors (TFTs) are widely used as switching devices ordriving elements of active-matrix display devices such as liquid crystaldisplay devices and organic electroluminescence (EL) display devices.

In recent years, TFTs that include channel layers composed of oxidesemiconductors such as zinc oxide (ZnO), indium gallium oxide (InGaO),and indium gallium zinc oxide (InGaZnO) have been actively researchedand developed. TFTs that contain oxide semiconductors in channel layersfeature low OFF current and high carrier mobility even in an amorphousstate and can be manufactured through low-temperature processes.

Japanese Unexamined Patent Application Publication No. 2007-311404discloses a technique for producing TFTs having stable characteristics.According to this technique, a heat treatment is performed afterformation of an oxide semiconductor film. International Publication No.2012/090490 discloses a technique of performing a plasma treatment suchas an ozone treatment on an oxide semiconductor film after formation ofthe oxide semiconductor film and before formation a protective film. Italso discloses that this plasma treatment suppresses occurrence ofoxygen defects in the oxide semiconductor film.

In forming thin film semiconductor devices by the techniques describedabove, a plasma treatment is performed on an oxide semiconductor film tosuppress occurrence of oxygen defects; however, the conditions of theplasma treatment are not disclosed. Thus, the oxide semiconductor may bedamaged by the plasma treatment and the characteristics may becomedegraded depending on the conditions.

SUMMARY

This disclosure provides a thin film semiconductor device that hasstable characteristics and a method for manufacturing the thin filmsemiconductor device.

Additional benefits and advantages of the disclosed embodiments will beapparent from the specification and figures. The benefits and/oradvantages may be individually provided by the various embodiments andfeatures of the specification and drawings disclosure, and need not allbe provided in order to obtain one or more of the same.

An aspect of the disclosure provides a thin film semiconductor devicethat comprises a substrate, a gate electrode disposed above thesubstrate, an oxide semiconductor layer disposed above the substrate soas to oppose the gate electrode, a first insulating layer disposed onthe oxide semiconductor layer, and a source electrode and a drainelectrode each connected to the oxide semiconductor layer. A density ofstates DOS [eV⁻¹cm⁻³] of oxygen defects in the oxide semiconductor layersatisfies the following relationship:DOS≦1.710×10¹⁷×(Ec−E)²−6.468×10¹⁷×(Ec−E)+6.113×10¹⁷provided that 2.0 eV≦Ec−E≦2.7 eVwhere Ec [eV] is an energy level of a conduction band edge of the oxidesemiconductor layer and E [eV] is a predetermined energy level of theoxide semiconductor layer.

These general and specific aspects may be implemented using a process,an electronic device, a system, and an integrated circuit, and anycombination of a process, an electronic device, a system, and anintegrated circuit.

According to this disclosure, a thin film semiconductor device that hasmore stable characteristics, and a method for manufacturing the thinfilm semiconductor device can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic cross-sectional views illustrating amethod for manufacturing a typical thin film semiconductor device.

FIG. 2 is an energy band diagram describing a problem caused by oxygendefects.

FIG. 3 is a graph showing a mobility-voltage characteristic of a typicalthin film semiconductor device before and after application of stress

FIG. 4 is a perspective partially cutaway view of an organic EL displaydevice according to a first embodiment.

FIG. 5 is an electric circuit diagram showing the configuration of apixel circuit in the organic EL display device according to the firstembodiment.

FIG. 6 is a schematic cross-sectional view of the thin filmsemiconductor device according to the first embodiment.

FIGS. 7A to 7G are schematic cross-sectional views illustrating a methodfor manufacturing a thin film semiconductor device according to thefirst embodiment.

FIG. 8A is a graph showing transmission characteristics of a thin filmsemiconductor device according to Comparative Example.

FIG. 8B is a graph showing transmission characteristics of a thin filmsemiconductor device according to the first embodiment.

FIG. 8C is a graph showing transmission characteristics of a thin filmsemiconductor device according to the first embodiment.

FIG. 9 is a graph showing a density of states of oxygen defects in anoxide semiconductor layer of a thin film semiconductor device accordingto the first embodiment.

FIG. 10A is a graph showing transmission characteristics of a thin filmsemiconductor device according to Comparative Example before and afterapplication of stress.

FIG. 10B is a graph showing transmission characteristics of a thin filmsemiconductor device according to the first embodiment before and afterapplication of stress.

FIG. 11 is a graph showing a relationship between a power density of aplasma treatment and a threshold voltage according to the firstembodiment.

FIG. 12 is a graph showing mobility-voltage characteristics of a thinfilm semiconductor device according to the first embodiment.

FIG. 13 is a graph showing a relationship between pressure conditions ofa plasma treatment and mobility according to the first embodiment.

FIG. 14 is a schematic cross-sectional view of a thin film semiconductordevice according to a second embodiment.

FIGS. 15A to 15H are schematic cross-sectional views illustrating amethod for manufacturing a thin film semiconductor device according tothe second embodiment.

FIG. 16 is a schematic cross-sectional view of a thin film semiconductordevice according to a third embodiment.

FIGS. 17A to 17G are schematic cross-sectional views illustrating amethod for manufacturing a thin film semiconductor device according tothe third embodiment.

DESCRIPTION OF EMBODIMENTS Findings that Form a Basis of this Disclosure

A typical thin film semiconductor device has been found to have thefollowing drawbacks.

FIGS. 1A and 1B are schematic cross-sectional views illustrating amethod for manufacturing a typical thin film semiconductor device.

Referring to FIG. 1A, a gate electrode 420 is formed on a substrate 410and a gate insulating film 430 is formed on the gate electrode 420. Apatterned oxide semiconductor layer 440 is formed on the gate insulatingfilm 430. In particular, an InGaZnO layer is formed on the gateinsulating film 430 by a sputtering method or the like and thenpatterned by a wet etching method. The oxide semiconductor layer 440 isformed in this way.

Referring to FIG. 1B, a channel protection layer 450 is formed on thegate insulating film 430 so as to cover the oxide semiconductor layer440. In particular, a silicon oxide film is formed by a plasma chemicalvapor deposition (CVD) method on the gate insulating film 430 so as tocover the oxide semiconductor layer 440. The channel protection layer450 is formed in this way.

During this process, as shown in FIG. 1B, oxygen defects occur in theoxide semiconductor layer 440 due to process damage. In particular,oxygen defects occur in the oxide semiconductor layer 440 due to wetetching at the time of the formation of the oxide semiconductor layer440 or due to a plasma treatment at the time of the formation of thechannel protection layer 450.

FIG. 2 is an energy band diagram describing a problem caused by oxygendefects.

As shown in FIG. 2, oxygen defects in an oxide semiconductor layerbecome a source of carriers. In particular, electrons and holes aregenerated once heat or light energy is applied to the oxidesemiconductor layer.

When a negative voltage is applied to a gate electrode, holes generatedin the oxide semiconductor layer are attracted to the negative voltagein the gate electrode and become captured in the gate insulating layer.As a result, a negative shift in threshold voltage occurs. Accordingly,for example, when this thin film semiconductor device is used as a drivetransistor of a display, this negative shift in threshold voltage causesluminance variation.

Oxygen defects in the oxide semiconductor layer decrease the resistancein a back channel-side region (hereinafter referred to as a back channelregion) of the oxide semiconductor layer. Carriers move about in theback channel region with a decreased resistance and the current in theoxide semiconductor layer increases as a result. The back channel regiondiscussed here is a region in the oxide semiconductor layer on theopposite side of the gate electrode (in FIG. 1B, the upper region of theoxide semiconductor layer 440).

FIG. 3 is a graph showing a mobility-voltage characteristic of a typicalthin film semiconductor device before and after application of stress.

Because the resistance in the back channel region is decreased, as shownin FIG. 3, the plot of the mobility against the gate-source voltage hasa peak (hereinafter referred to as a mobility curve peak) if thegate-source voltage (Vgs) is lower than the drain-source voltage (Vds).The mobility curve peak changes greatly between before and afterapplication of stress, as shown in FIG. 3. Accordingly, for example,when this thin film semiconductor device is used as a drive transistorof a display, the change in the mobility curve peak causes luminancevariation.

As described above, a typical thin film semiconductor device cannotexhibit stable characteristics because of occurrence of oxygen defects.

BRIEF OVERVIEW OF THE DISCLOSURE

A thin film semiconductor device according to this disclosure comprisesa substrate, a gate electrode disposed above the substrate, an oxidesemiconductor layer disposed above the substrate so as to oppose thegate electrode, a first insulating layer disposed on the oxidesemiconductor layer, and a source electrode and a drain electrode eachconnected to the oxide semiconductor layer, in which a density of statesDOS [eV⁻¹cm⁻³] of oxygen defects in the oxide semiconductor layersatisfies the following relationship:DOS≦1.710×10¹⁷×(Ec−E)²−6.468×10¹⁷×(Ec−E)+6.113×10¹⁷provided that 2.0 eV≦Ec−E≦2.7 eVwhere Ec [eV] is an energy level of a conduction band edge of the oxidesemiconductor layer and E [eV] is a predetermined energy level of theoxide semiconductor layer.

Since the thin film semiconductor device includes an oxide semiconductorlayer having a low density of states of oxygen defects, the negativeshift in threshold voltage is decreased and the mobility curve peak isdecreased. Accordingly, the thin film semiconductor device exhibits morestable characteristics.

In the thin film semiconductor device according to this disclosure, thedensity of states DOS [eV⁻¹cm⁻³] may satisfy the following relationship:DOS≦1.332×10¹⁰×(Ec−E)^(14.65)provided that 2.0 eV≦Ec−E≦2.7 eV.

Since the thin film semiconductor device includes an oxide semiconductorlayer having an even lower low density of states of oxygen defects, thenegative shift in threshold voltage is further decreased and themobility curve peak is further decreased. Accordingly, the thin filmsemiconductor device exhibits more stable characteristics.

The thin film semiconductor device according to this disclosure mayfurther comprise a second insulating layer disposed on the gateelectrode. The oxide semiconductor layer may be disposed on the secondinsulating layer. A contact hole for exposing part of the oxidesemiconductor layer may be disposed in the first insulating layer. Thesource electrode and the drain electrode may be disposed on the firstinsulating layer and connected to the oxide semiconductor layer throughthe contact hole.

According to this structure, the thin film semiconductor device can beused as a bottom-gate, channel-protective thin film transistor andexhibits more stable characteristics.

The thin film semiconductor device according to this disclosure mayfurther comprise a second insulating layer formed on the gate electrode.The oxide semiconductor layer may be disposed on the second insulatinglayer. The source electrode and the drain electrode may be disposed onthe oxide semiconductor layer. The first insulating layer may bedisposed on the source electrode, the drain electrode, and the oxidesemiconductor layer.

According to this structure, the thin film semiconductor device can beused as a bottom-gate, channel-etch thin film transistor and exhibitsmore stable characteristics.

In the thin film semiconductor device according to this disclosure, thegate electrode may be disposed on the first insulating layer.

According to this structure, the thin film semiconductor device can beused as a top-gate thin film transistor and exhibits more stablecharacteristics.

In the thin film semiconductor device according to this disclosure, theoxide semiconductor layer may comprise a transparent amorphous oxidesemiconductor.

Since the oxide semiconductor layer includes a transparent amorphousoxide semiconductor, the carrier mobility can be increased.

A method for manufacturing a thin film semiconductor device according tothis disclosure comprises steps of: (a) forming a gate electrode above asubstrate, (b) forming an oxide semiconductor layer at a position abovethe substrate and opposing the gate electrode, (c) performing a plasmatreatment on the oxide semiconductor layer by using gas containingoxygen, (d) forming a first insulating layer on the oxide semiconductorlayer, and (e) forming a source electrode and a drain electrode eachconnected to the oxide semiconductor layer. In the performing step (c)the plasma treatment is performed in particular conditions that adensity of states DOS [eV⁻¹cm⁻³] of oxygen defects in the oxidesemiconductor layer satisfies the following relationship:DOS≦1.710×10¹⁷×(Ec−E)²−6.468×10¹⁷×(Ec−E)+6.113×10¹⁷provided that 2.0 eV≦Ec−E≦2.7 eV where Ec [eV] is an energy level of aconduction band edge of the oxide semiconductor layer and E [eV] is apredetermined energy level of the oxide semiconductor layer.

According to this method, since the density of states of oxygen defectsin the oxide semiconductor layer is low, the negative shift in thresholdvoltage can be decreased and the mobility curve peak can be decreased.Accordingly, a thin film semiconductor device having more stablecharacteristics can be manufactured.

In the method for manufacturing a thin film semiconductor deviceaccording to this disclosure, the gas may contain nitrous, oxide and apower density of the plasma treatment may be 1 [W/cm²] or less.

According to this method, an oxide semiconductor layer having asufficiently low density of states of oxygen defects can be formed byperforming a plasma treatment at a power density of 1 [W/cm²] or lessand thus the negative shift in threshold voltage can be decreased.

In the method for manufacturing a thin film semiconductor deviceaccording to this disclosure, the power density of the plasma treatmentmay be 0.2 [W/cm²] or more.

According to this method, an oxide semiconductor layer having asufficiently low density of states of oxygen defects can be formed byperforming a plasma treatment at a power density of 0.2 [W/cm²] or moreand thus the negative shift in threshold voltage can be decreased.

In the method for manufacturing a thin film semiconductor deviceaccording to this disclosure, the gas may contain nitrous oxide, and inthe performing step (c) the plasma treatment may be performed at apressure of 2.0 [Torr] or more.

According to this method, an oxide semiconductor layer having asufficiently low density of states of oxygen defects can be formed byperforming a plasma treatment at a pressure of 2.0 [Torr] or more andthus the mobility curve peak can be decreased.

The method for manufacturing a thin film semiconductor device accordingto this disclosure may further comprise a step of: (f) forming a secondinsulating layer on the gate electrode. In the forming step (b) theoxide semiconductor layer may be formed on the second insulating layer.In the forming step (d) the first insulating layer may be formed so thatpart of the oxide semiconductor layer is exposed. In the forming step(e) the source electrode and the drain electrode may be formed so as toconnect to the oxide semiconductor layer at the exposed part.

According to this method, a thin film semiconductor device which can beused as a bottom-gate, channel-protective thin film transistor havingmore stable characteristics can be manufactured.

The method for manufacturing a thin film semiconductor device accordingto this disclosure may further comprise a step of: (f) forming a secondinsulating layer on the gate electrode. In the forming step (b) theoxide semiconductor layer may be formed on the second insulating layer.In the forming step (e) the source electrode and the drain electrode maybe formed on the oxide semiconductor layer. In the forming step (d) thefirst insulating layer may be formed on the source electrode, the drainelectrode, and the oxide semiconductor layer.

According to this method, a thin film semiconductor device which can beused as a bottom-gate, channel-etch thin film transistor having morestable characteristics can be manufactured.

In the method for manufacturing a thin film semiconductor deviceaccording to this disclosure, the performing step (c) may be performedbefore the forming step (e).

According to this method in which a plasma treatment is performed beforeforming a source electrode and a drain electrode, a source electrode, adrain electrode, and a first insulating layer can be formed after thedensity of states of oxygen defects in the oxide semiconductor layer isdecreased. Accordingly, a thin film semiconductor device having morestable characteristics can be manufactured.

In the forming a gate electrode of the method for manufacturing a thinfilm semiconductor device according to this disclosure, in the formingstep (a) the gate electrode may be formed on the first insulating layer.

According to this method, a thin film semiconductor device that can beused as a top-gate thin film transistor having more stablecharacteristics can be manufactured.

In the method for manufacturing a thin film semiconductor deviceaccording to this disclosure, the performing step (c) may be performedbefore the forming step (d).

According to this method in which a plasma treatment is performed beforeforming a first insulating layer, a first insulating layer can be formedafter the density of states of oxygen defects in the oxide semiconductorlayer is decreased. Moreover, occurrence of oxygen defects duringformation of the first insulating layer can be suppressed.

In the method for manufacturing a thin film semiconductor deviceaccording to this disclosure, the oxide semiconductor layer may comprisea transparent amorphous oxide semiconductor.

Since the oxide semiconductor layer is a transparent amorphous oxidesemiconductor layer, a thin film semiconductor device having a highcarrier mobility can be manufactured.

Embodiments of a thin film semiconductor device, a method formanufacturing the thin film semiconductor device, and an organic ELdisplay device that includes the thin film semiconductor device will nowbe described with reference to the drawings. Note that the embodimentsdescribed below are merely some specific examples of this disclosure.The numerals, forms, materials, constitutional elements, positions andconnecting forms of constitutional elements, steps, the order in whichthe steps are performed, etc., are merely illustrative and do not limitthe scope of the present invention. Accordingly, among theconstitutional elements described in the embodiments below, those whichare not described in the independent claims that define the highestconcept of the present invention are described as optional elements.

It should also be noted that the drawings are merely schematic views anddo not necessarily present accurate illustrations. In the drawings,structures that are substantially identical are given the same referencecharacters and description therefor is simplified or omitted to avoidredundancy.

First Embodiment Organic EL Display Device

First, a structure of an organic EL display device 10 according to afirst embodiment is described with reference to FIG. 4. FIG. 4 is aperspective partially cutaway view of an organic EL display deviceaccording to this embodiment.

As shown in FIG. 4, the organic EL display device 10 has a multilayeredstructure that includes a TFT substrate (TFT array substrate) 20 inwhich plural thin film transistors are arranged and organic EL elements(emitting portions) 40. The organic EL elements each have an anode 41which is a lower electrode, an EL layer 42 which is an emitting layercomposed of an organic material, and a cathode 43 which is a transparentupper electrode.

In the TFT substrate 20, plural pixels 30 are arranged in a matrix andone pixel circuit 31 is provided for each of the pixels 30.

The organic EL elements 40 are formed to respectively correspond to thepixels 30. The pixel circuit 31 of each of the pixels 30 controlsemission of the corresponding organic EL element 40. The organic ELelements 40 are formed on an interlayer dielectric film (planarizingfilm) that covers the thin film transistors.

The organic EL element 40 has a structure in which the EL layer 42 isinterposed between the anode 41 and the cathode 43. A hole transportlayer is stacked between the anode 41 and the EL layer 42 and anelectron transport layer is stacked between the EL layer 42 and thecathode 43. An additional organic functional layer may be disposedbetween the anode 41 and the cathode 43.

Each of the pixels 30 is driven and controlled by the correspondingpixel circuit 31. In the TFT substrate 20, plural gate wires (scanningwires) 50, plural source wires (signal wires) 60, and plural powersupply wires (not shown in FIG. 4) are formed. The plural gate wires 50extend in the row direction of the pixels 30. The plural source wires 60intersect the gate wires 50 and are arranged in the column direction ofthe pixels 30. The plural power supply wires are arranged in parallel tothe source wires 60. Gate wires 50 and source wires 60 that intersecteach other define each of the pixels 30.

A gate wire 50 is connected to a row of gate electrodes of the thin filmtransistors that work as switching elements in the pixel circuits 31. Asource wire 60 is connected to a column of source electrodes of the thinfilm transistors that work as switching elements in the pixel circuits31. A power source wire is connected to a column of drain electrodes ofthe thin film transistors that work as drive elements in the pixelcircuits 31.

The circuit configuration of the pixel circuit 31 of the pixel 30 willnow be described with reference to FIG. 5. FIG. 5 is an electric circuitdiagram showing the configuration of a pixel circuit of the organic ELdisplay device of this embodiment.

As shown in FIG. 5, the pixel circuit 31 includes a thin film transistor32 that functions as a drive element, a thin film transistor 33 thatfunctions as a switching element, and a capacitor 34 that stores datafor performing display in the corresponding pixel 30. In thisembodiment, the thin film transistor 32 is a drive transistor fordriving the organic EL element 40 and the thin film transistor 33 is aswitching transistor for selecting a pixel 30.

The thin film transistor 32 includes a gate electrode 32 g, a drainelectrode 32 d, a source electrode 32 s, and a semiconductor film (notshown in the drawing). The gate electrode 32 g is connected to the drainelectrode 33 d of the thin film transistor 33 and one end of thecapacitor 34. The drain electrode 32 d is connected to a power supplywire 70 and the other end of the capacitor 34. The source electrode 32 sis connected to the anode 41 of the organic EL element 40.

The thin film transistor 32 supplies a current that corresponds to thedata voltage held in the capacitor 34 to the anode 41 of the organic ELelement 40 from the power supply wire 70 via the source electrode 32 s.As a result, a drive current flows from the anode 41 to the cathode 43in the organic EL element 40 and the EL layer 42 emits light.

The thin film transistor 33 includes a gate electrode 33 g, a sourceelectrode 33 s, a drain electrode 33 d, and a semiconductor film (notshown in the drawing). The gate electrode 33 g is connected to the gatewire 50. The source electrode 33 s is connected to the source wire 60.The drain electrode 33 d is connected to one end of the capacitor 34 andthe gate electrode 32 g of the thin film transistor 32. When aparticular voltage is applied to the gate wire 50 and the source wire 60connected to this thin film transistor 33, the voltage applied to thesource wire 60 is stored in the capacitor 34 as a data voltage.

Note that in the organic EL display device 10 having the aforementionedconfiguration, an active matrix system is employed. The active matrixsystem performs display control based on each of the pixels 30 locatedat the intersections of the gate wires 50 and the source wires 60. Underthis system, the thin film transistors 32 and 33 of the pixels 30(sub-pixels R, G, and B) are operated to cause the corresponding organicEL elements 40 to selectively emit light and display a desired image.

Thin Film Semiconductor Device

A thin film semiconductor device according to this embodiment will nowbe described. A thin film semiconductor device of this embodiment is abottom-gate, channel-protective thin film transistor.

FIG. 6 is a schematic cross-sectional view of a thin film semiconductordevice according to this embodiment.

Referring to FIG. 6, a thin film semiconductor device 100 according tothis embodiment includes a substrate 110, a gate electrode 120, a gateinsulating film 130, an oxide semiconductor layer 140, a channelprotective layer 150, a source electrode 160 s, and a drain electrode160 d. The oxide semiconductor layer 140 includes an oxygen rich layer141.

The thin film semiconductor device 100 is, for example, a thin filmtransistor 32 or thin film transistor 33 shown in FIG. 5. In otherwords, the thin film semiconductor device 100 can be used as either adrive transistor or a switching transistor. In the case where the thinfilm semiconductor device 100 is a thin film transistor 32, the gateelectrode 120 corresponds to the gate electrode 32 g, the sourceelectrode 160 s corresponds to the source electrode 32 s, and the drainelectrode 160 d corresponds to the drain electrode 32 d. In the casewhere the thin film semiconductor device 100 is a thin film transistor33, the gate electrode 120 corresponds to the gate electrode 33 g, thesource electrode 160 s corresponds to the source electrode 33 s, and thedrain electrode 160 d corresponds to the drain electrode 33 d.

The substrate 110 is a substrate composed of a material having anelectric insulation property. For example, the substrate 110 is asubstrate composed of a glass material, such as alkali-free glass,quartz glass, or high-heat-resistance glass, a substrate composed of aresin material such as polyethylene, polypropylene, or polyimide, asubstrate composed of a semiconductor material such as silicon orgallium arsenide, or a substrate composed of a metal material such asstainless steel coated with an insulating layer.

The substrate 110 may be a flexible substrate such as a resin substrate.In such a case, the thin film semiconductor device 100 can be used as aflexible display.

The gate electrode 120 is formed on the substrate 110 and has apredetermined form. The gate electrode 120 is an electrode composed ofan electrically conductive material. Examples of the material for thegate electrode 120 include metals such as molybdenum, aluminum, copper,tungsten, titanium, manganese, chromium, tantalum, niobium, silver,gold, platinum, palladium, indium, nickel, and neodymium, alloys ofthese, conductive metal oxides such as indium tin oxide (ITO),aluminum-doped zinc oxide (AZO), and gallium-doped zinc oxide (GZO), andconductive polymers such as polythiophene and polyacetylene. The gateelectrode 120 may have a multilayered structure formed by stacking thesematerials.

The gate insulating film 130 is an example of a second insulating layerformed on the gate electrode 120. The gate insulating film 130 is formedon the gate electrode 120 and the substrate 110 so as to cover the gateelectrode 120.

The gate insulating film 130 is composed of a material having anelectric insulation property. For example, the gate insulating film 130is a single-layer film such as a silicon oxide film, a silicon nitridefilm, a silicon oxynitride film, an aluminum oxide film, a tantalumoxide film, or a hafnium oxide film, or a stacked film of these films.

The oxide semiconductor layer 140 is formed above the substrate 110 soas to oppose the gate electrode 120. In particular, the oxidesemiconductor layer 140 is formed at a position that opposes the gateelectrode 120 and on the gate insulating film 130. For example, an oxidesemiconductor layer 140 having an island form is formed on the gateinsulating film 130 and above the gate electrode 120.

An oxide semiconductor material that contains at least one selected fromindium (In), gallium (Ga), and zinc (Zn) is used as the material for theoxide semiconductor layer 140. For example, the oxide semiconductorlayer 140 is composed of a transparent amorphous oxide semiconductor(TAOS) such as amorphous indium gallium zinc oxide (InGaZnO:IGZO). TheIn:Ga:Zn ratio is, for example, about 1:1:1. The In:Ga:Zn ratio may be,but is not limited to be, in the range of 0.8 to 1.2:0.8 to 1.2:0.8 to1.2.

A thin film transistor that includes a channel layer composed of atransparent amorphous oxide semiconductor has a high carrier mobilityand is suitable for use in a large-screen, high-definition displaydevices. Since a transparent amorphous oxide semiconductor can be formedinto a film at low temperature, it can be easily formed on a flexiblesubstrate such as a plastic or a film.

As shown in FIG. 6, the oxygen rich layer 141 is formed in the backchannel region of the oxide semiconductor layer 140. The oxygen richlayer 141 is a region having fewer oxygen defects among the oxidesemiconductor layer 140 and is formed by a plasma treatment.

The details of the oxide semiconductor layer 140 are described later.

The channel protective layer 150 is one example of a first insulatinglayer formed on the oxide semiconductor layer 140. For example, thechannel protective layer 150 is formed on the oxide semiconductor layer140 and the gate insulating film 130 so as to cover the oxidesemiconductor layer 140.

Holes are formed in part of the channel protective layer 150 so as topenetrate through the channel protective layer 150. In other words,contact holes for exposing part of the oxide semiconductor layer 140(oxygen rich layer 141) are formed in the channel protective layer 150.The oxide semiconductor layer 140 is connected to the source electrode160 s and the drain electrode 160 d through the opening portions(contact hole).

The channel protective layer 150 is composed of a material having anelectric insulation property. For example, the channel protective layer150 is a single-layer film, e.g., a film composed of an inorganicmaterial, such as a silicon oxide film, a silicon nitride film, asilicon oxynitride film, or an aluminum oxide film or a film composed ofan inorganic material containing silicon, oxygen, and carbon, or amultilayered film that includes these films.

The source electrode 160 s and the drain electrode 160 d are formed onthe channel protective layer 150 and each has a predetermined form. Inparticular, the source electrode 160 s and the drain electrode 160 d areconnected to the oxide semiconductor layer 140 (oxygen rich layer 141)through the contact holes formed in the channel protective layer 150.The source electrode 160 s and the drain electrode 160 d are arranged tooppose each other on the channel protective layer 150 so as to be spacedfrom each other in a direction horizontal to the substrate.

The source electrode 160 s and the drain electrode 160 d are each anelectrode composed of an electrically conductive material. The materialfor the source electrode 160 s and the drain electrode 160 d is, forexample, the same as those for the gate electrode 120.

Method for Manufacturing Thin Film Semiconductor Device

Next, a method for manufacturing a thin film semiconductor deviceaccording to this embodiment is described with reference to FIGS. 7A to7G. FIGS. 7A to 7G are schematic cross-sectional views illustrating amethod for manufacturing a thin film semiconductor device according tothis embodiment.

First, as shown in FIG. 7A, a substrate 110 is prepared and a gateelectrode 120 having a predetermined form is formed above the substrate110. For example, a metal film is formed on the substrate 110 by asputtering method. And the metal film is processed by a photolithographymethod and a wet etching method to form a gate electrode 120 having apredetermined form.

To be more specific, first, a glass substrate is prepared as thesubstrate 110. A molybdenum film (Mo film) and a copper film (Cu film)are sequentially formed on the substrate 110 by a sputtering method.Then the Mo film and the Cu film are patterned by a photolithographymethod and a wet etching method so as to form a gate electrode 120. Thethickness of the gate electrode 120 is, for example, 20 nm to 500 nm.The Mo film and the Cu film may be wet-etched by using, for example, achemical solution containing aqueous hydrogen peroxide (H₂O₂) and anorganic acid.

Next, as shown in FIG. 7B, a gate insulating film 130 is formed abovethe substrate 110. For example, a gate insulating film 130 is formed onthe substrate 110 and the gate electrode 120 by a plasma CVD method.

In particular, a silicon nitride film and a silicon oxide film aresequentially formed by a plasma CVD method on the substrate 110 so as tocover the gate electrode 120. As a result, a gate insulating film 130 isformed. The thickness of the gate insulating film 130 is, for example,50 nm to 300 nm.

The silicon nitride film can be formed by using, for example, a silanegas (SiH₄), ammonia gas (NH₃), and nitrogen gas (N₂) as the introducedgas. The silicon oxide film can be formed by using silane gas (SiH₄) andnitrous oxide gas (N₂O) as the introduced gas.

Next, as shown in FIG. 7C, an oxide semiconductor film 140 a is formedat a position above the substrate 110 and opposing the gate electrode120. For example, an oxide semiconductor film 140 a is formed on thegate insulating film 130 by a sputtering method. The thickness of theoxide semiconductor film 140 a is, for example, 20 to 200 nm.

In particular, an amorphous InGaZnO film is formed on the gateinsulating film 130 by a sputtering method using a target materialhaving an In:Ga:Zn compositional ratio of 1:1:1 in an oxygen atmosphere.

Next, as shown in FIG. 7D, an oxide semiconductor layer 140 having apredetermined form is formed above the substrate 110. For example, theoxide semiconductor film 140 a is patterned so as to form an oxidesemiconductor layer 140 on the gate insulating film 130.

In particular, an amorphous InGaZnO film formed on the gate insulatingfilm 130 is patterned by a photolithography method and a wet etchingmethod so as to form an oxide semiconductor layer 140. The thickness ofthe oxide semiconductor layer 140 is, for example, 20 to 200 nm. TheInGaZnO film may be wet-etched by using a chemical solution containing,for example, phosphoric acid (H₃PO₄), nitric acid (HNO₃), acetic acid(CH₃COOH), and water.

Next, as shown in FIG. 7E, a plasma treatment is performed by usingoxygen-containing gas. For example, an oxygen rich layer 141 is formedin the oxide semiconductor layer 140 by exposing the oxide semiconductorlayer 140 to plasma 170. The thickness of the oxygen rich layer 141 is,for example, 5 to 50 nm.

In particular, a plasma treatment is performed on the oxidesemiconductor layer 140 by using nitrous oxide gas (N₂O) as theintroduced gas at a power density of 0.2 [W/cm²] or more and 1 [W/cm²]or less and under pressure of 2.0 [Torr] or more. The time for theplasma treatment is, for example, 30 seconds to 5 minutes. Theconditions for the plasma treatment are not limited to these. Forexample, the introduced gas may be oxygen gas (O₂), ozone gas (O₃),nitrogen dioxide gas (NO₂), or the like.

The region of the oxide semiconductor layer 140 exposed to the plasma bythe plasma treatment forms an oxygen rich layer 141. As shown in FIG.7E, the oxygen rich layer 141 is formed in the back channel region ofthe oxide semiconductor layer 140.

Next, as shown in FIG. 7F, a channel protective layer 150 is formed onthe oxide semiconductor layer 140. For example, a channel protectivelayer 150 is formed on the oxide semiconductor layer 140 and the gateinsulating film 130 so as to cover the oxide semiconductor layer 140(oxygen rich layer 141).

In particular, a silicon oxide film is formed on the gate insulatingfilm 130 by a plasma CVD method. A channel protective layer 150 isformed in this way. The thickness of the channel protective layer 150is, for example, 50 to 500 nm.

The channel protective layer 150 is then patterned into a predeterminedform. In particular, contact holes are formed in the channel protectivelayer 150 so as to expose part of the oxide semiconductor layer 140(oxygen rich layer 141). For example, part of the channel protectivelayer 150 is etched away to form contact holes.

To be more specific, first, part of the channel protective layer 150 isetched away by a photolithography method and a dry etching method. As aresult of this etching, contact holes are formed in regions that willform a source contact region and a drain contact region in the oxidesemiconductor layer 140. For example, when the channel protective layer150 is a silicon oxide film, a reactive ion etching (RIE) method can beemployed as the dry etching method. The etching gas that can be usedhere is, for example, carbon tetrafluoride (CF₄) and oxygen gas (O₂).The parameters such as gas flow rate, pressure, applied power, andfrequency are appropriately set in accordance with the substrate sizeand the thickness of the film to be etched.

Next, as shown in FIG. 7G, a source electrode 160 s and a drainelectrode 160 d connected to the oxide semiconductor layer 140 areformed. For example, a source electrode 160 s having a predeterminedform and a drain electrode 160 d having a predetermined form are formedon the channel protective layer 150 while filling the contact holesformed in the channel protective layer 150.

In particular, a source electrode 160 s and a drain electrode 160 d thatare spaced from each other are formed on the channel protective layer150 and in the contact holes. To be more specific, a Mo film, a Cu film,and a CuMn film are sequentially formed by a sputtering method on thechannel protective layer 150 and in the contact holes. Then, the Mofilm, the Cu film, and the CuMn film are patterned by a photolithographymethod and a wet etching method so as to form a source electrode 160 sand a drain electrode 160 d.

The thickness of the source electrode 160 s and the drain electrode 160d is, for example, 100 nm to 500 nm. The Mo film, the Cu film, and theCuMn film can be wet-etched by using, for example, a chemical solutioncontaining aqueous hydrogen peroxide (H₂O₂) and an organic acid.

As a result of performing the above-described processes, a thin filmsemiconductor device 100 is manufactured.

Relationship Between Density of States of Oxygen Defects and PlasmaTreatment

First, a method for measuring the density of states of oxygen defects inoxide semiconductor layer 140 is described.

When transmission characteristics are measured by applying light to IGZOused as a material for the oxide semiconductor layer 140, an increase incurrent in an OFF region of the transmission characteristics and anegative shift in threshold voltage are observed. This is because oxygendefects (Vo) forming an energy level near a valence band in the band gapform divalent cations (Vo²⁺) that form energy level near the conductionband by irradiation with light and release two electrons to theconduction band.

The amount of the negative shift in threshold voltage is dependent onthe amount of oxygen defects in the band gap. The larger the amount ofoxygen defects, the larger the amount of the shift in threshold voltage.Moreover, the energy of the applied light corresponds to the depth ofthe energy level of the oxygen defects (Vo) from the conduction band.

According to the method for measuring the density of states of oxygendefects, the transmission characteristics are first measured in a darkstate. During this process, the transmission characteristics can beacquired by, for example, performing a sweep at a voltage step of 0.1 Vin the gate-source voltage Vgs range of from −15 V to +15 V at adrain-source voltage Vds set to 4.1 V.

Next, the transmission characteristics under light irradiation (brightstate) are evaluated. For example, light having an energy E of 2.0 eV,2.3 eV, and 2.7 eV is applied to the oxide semiconductor layer 140 at aluminance of 100 μW/cm². Evaluation is sequentially performed from anoxygen defect level that exists at an energy level relatively shallowfrom the lower edge of the conduction band and having a relatively smalloptical response up to an oxygen defect level that exists at a deeperenergy level. In order to perform the evaluation, the energy of lightapplied is increased stepwise from 2.0 eV to 2.3 eV and to 2.7 eV tomeasure the transmission characteristics under light irradiation at eachlight energy.

Then the threshold voltages Vth are calculated from the transmissioncharacteristics obtained in the dark state and bright state. Thethreshold Vth is defined as the value of Vgs at the time when thedrain-source current Ids satisfies Ids=(W/L)×1 nA where W represents achannel width and L represents a channel length. After normalized Vth inthe dark state to the bright state are calculated, the density of statesDOS [eV⁻¹cm⁻³] of oxygen defects is calculated by using formula (1):

$\begin{matrix}{{D\; O\; S} = {\left( \frac{C}{q\; t} \right)\left( \frac{\Delta\; V_{th}}{\Delta\; E} \right)}} & {{Formula}\mspace{14mu}(1)}\end{matrix}$

In the formula, C represents a gate insulating film capacitance per unitarea, q represents an elementary charge t represents the thickness of asemiconductor film (thickness of the IGZO film), ΔVth represents thedifference in Vth obtained in the respective measurements, for example,the difference between Vth obtained in the dark state and Vth obtainedin the bright state, and ΔE represents the difference in light energyapplied during measurement of the respective transmissioncharacteristics, for example, the difference between the light energy Ein the dark state and the light energy E in the bright state. The “darkstate” corresponds to the measurement performed at a light energy E of 0eV.

The relationship between the density of states of oxygen defects in theoxide semiconductor layer 140 and the plasma treatment will now bedescribed with reference to FIGS. 8A to 9.

According to this embodiment, a resist is applied to a particular regionon the oxide semiconductor film 140 a and etching is performed so as toform the oxide semiconductor layer 140. Accordingly, the upper surfaceof the oxide semiconductor layer 140 is damaged by the resist and oxygendefects occur in the region that includes the upper surface of the oxidesemiconductor layer 140. In particular, oxygen defects occur in the backchannel region.

FIG. 8A is a graph showing transmission characteristics of a thin filmsemiconductor device according to Comparative Example. The thin filmsemiconductor device according to Comparative Example is identical tothe thin film semiconductor device of this embodiment except that aplasma treatment is not performed on the oxide semiconductor layer.

As shown in FIG. 8A, the transmission characteristics greatly change asthe oxide semiconductor layer is irradiated with light. In particular,the drain-source current Ids against the same gate-source voltage Vgstends to increase when the light is applied than when light is notapplied. This is due to oxygen defects in the oxide semiconductor layeras discussed above. In other words, the larger the change intransmission characteristics induced by irradiation with light, thehigher the density of states of oxygen defects in the oxidesemiconductor layer.

In this embodiment, after the oxide semiconductor layer 140 is formedand before the channel protective layer 150 is formed, a plasmatreatment is performed by using gas containing nitrous oxide so as todecrease the oxygen defects in the oxide semiconductor layer 140.

FIGS. 8B and 8C are graphs showing transmission characteristics of thinfilm semiconductor devices according to this embodiment. That is, FIGS.8B and 8C show the transmission characteristics of thin filmsemiconductor devices 100 in which a plasma treatment is performed onthe oxide semiconductor layer 140. FIG. 8B shows the transmissioncharacteristics of a thin film semiconductor device 100 manufactured ata power density of 1 [W/cm²] by using nitrous oxide gas (N₂O) as theintroduced gas. FIG. 8C shows the transmission characteristics of a thinfilm semiconductor device 100 manufactured at a power density of 0.5[W/cm²] by using nitrous oxide gas (N₂O) as the introduced gas.

Comparison of FIGS. 8A and 8B shows that the change in transmissioncharacteristics that occurs when the oxide semiconductor layer isirradiated with light is suppressed by performing a plasma treatmentusing nitrous oxide gas (N₂O). The density of states of oxygen defectsin the oxide semiconductor layer can be decreased by performing a plasmatreatment using nitrous oxide gas (N₂O).

As shown in FIG. 8C, the change in transmission characteristics thatoccurs when the oxide semiconductor layer is irradiated with light isfurther suppressed by performing a plasma treatment at a lower powerdensity. The density of states of oxygen defects in the oxidesemiconductor layer can be further decreased by performing a plasmatreatment using nitrous oxide gas (N₂O) at a low power density.

To be more specific, the density of states of oxygen defects in a thinfilm semiconductor device manufactured at a power density of 0.5 [W/cm²]is lower than that in a thin film semiconductor device manufactured at apower density of 1 [W/cm²].

FIG. 9 is a graph showing the density of states of oxygen defects in anoxide semiconductor layer of a thin film semiconductor device accordingto this embodiment.

For example, the density of states DOS [eV⁻¹cm⁻³] of oxygen defects inthe oxide semiconductor layer 140 according to this embodiment satisfiesformula (2) where Ec [eV] is the energy level at the conduction bandedge of the oxide semiconductor layer 140 and E [eV] is a predeterminedenergy level of the oxide semiconductor layer 140. Note that formula (2)in this embodiment is in the range of 2.0≦Ec−E≦2.7.DOS≦1.710×10¹⁷×(Ec−E)²−6.468×10¹⁷×(Ec−E)+6.113×10¹⁷  Formula (2)

This formula indicates the region (region A) below the bold solid line(the line at the middle) in FIG. 9. The bold solid line in FIG. 9indicates the results of a plasma treatment performed on the oxidesemiconductor layer 140 at a power density of 1 [W/cm²] by using nitrousnitrogen gas (N₂O) as the introduced gas.

In particular, the density of states DOS [eV⁻¹ cm⁻²] of oxygen defectsin the oxide semiconductor layer 140 satisfies DOS [eV⁻¹cm⁻³]≦1.12×10¹⁷when the energy Ec−E from the conduction band edge is 2.7 eV. Thedensity of states DOS [eV⁻¹cm⁻³] of oxygen defects in the oxidesemiconductor layer 140 satisfies DOS [eV⁻¹ cm⁻³]≦2.82×10¹⁶ when theenergy Ec−E from the conduction band edge is 2.3 eV. The density ofstates DOS [eV⁻¹ cm⁻³] of oxygen defects in the oxide semiconductorlayer 140 satisfies DOS [eV⁻¹cm⁻³]≦1.70×10¹⁵ when the energy Ec−E fromthe conduction band edge is 2.0 eV.

The density of states DOS [eV⁻¹cm⁻³] of oxygen defects in the oxidesemiconductor layer 140 may satisfy formula (3) where Ec [eV] is theenergy level at the conduction band edge of the oxide semiconductorlayer 140 and E [eV] is a predetermined energy level of the oxidesemiconductor layer 140. In this embodiment, the formula (3) is in therange of 2.0≦Ec−E≦2.7.DOS≦1.332×10¹⁰×(Ec−E)^(14.65)  Formula (3)

This formula indicates the region (region B) below the broken line (theline at the bottom) in FIG. 9. The broken line in FIG. 9 indicates theresults of a plasma treatment performed on the oxide semiconductor layer140 at a power density of 0.5 [W/cm²] by using nitrous nitrogen gas(N₂O) as the introduced gas.

In particular, the density of states DOS [eV⁻¹cm⁻³] of oxygen defects inthe oxide semiconductor layer 140 satisfies DOS [eV⁻¹cm⁻³]≦2.78×10¹⁶when the energy Ec−E from the conduction band edge is 2.7 eV. Thedensity of states DOS [eV⁻¹cm⁻³] of oxygen defects in the oxidesemiconductor layer 140 satisfies DOS [eV⁻¹cm⁻³]≦2.65×10¹⁵ when theenergy Ec−E from the conduction band edge is 2.3 eV. The density ofstates DOS [eV⁻¹cm⁻³] of oxygen defects in the oxide semiconductor layer140 satisfies DOS [eV⁻¹cm⁻³]≦3.42×10¹⁴ when the energy Ec−E from theconduction band edge is 2.0 eV.

The dot line (line at the top) in FIG. 9 indicates the density of statesof oxygen defects in an oxide semiconductor layer not subjected to aplasma treatment. As shown in FIG. 9, the density of states of oxygendefects in the oxide semiconductor layer 140 can be decreased byperforming a plasma treatment. The density of states of oxygen defectsin the oxide semiconductor layer 140 can be further decreased bylowering the power density of the plasma treatment. Note that, asdiscussed above, the fact that a desired density of states of oxygendefects is obtained can be confirmed by evaluating transmissioncharacteristics of the oxide semiconductor layer 140 irradiated withlight (bright state).

Effect of Suppressing Decrease in Threshold Voltage

Next, the effect of suppressing a decrease in threshold voltage, whichis achieved in the thin film semiconductor device 100 according to thisembodiment, is described with reference to FIGS. 10A to 11.

FIG. 10A is a graph showing the transmission characteristics of a thinfilm semiconductor device according to Comparative Example before andafter application of stress. FIG. 10A shows the transmissioncharacteristics of a thin film semiconductor device in which the oxygensemiconductor layer is not subjected to a plasma treatment with nitrousoxide gas (N₂O).

FIG. 10B is a graph showing the transmission characteristics of a thinfilm semiconductor device according to this embodiment before and afterapplication of stress. FIG. 10B shows the transmission characteristicsof a thin film semiconductor device in which the oxygen semiconductorlayer 140 is subjected to a plasma treatment at a power density of 1[W/cm²] using nitrous oxide gas (N₂O) as the introduced gas.

The conditions of the stress applied in each case are as follows:gate-source voltage Vgs=−20 V, drain-source voltage Vds=0 V, temperatureT=90° C., and time t=2000 sec.

As shown in FIG. 10A, the threshold voltage of the thin filmsemiconductor device not subjected to a plasma treatment using nitrousoxide gas (N₂O) decreases after application of stress. In particular,the threshold voltage decreased by 2.6 V after the application of stressfrom the threshold voltage before application of stress.

In contrast, as shown in FIG. 10B, the threshold voltage of the thinfilm semiconductor device subjected to a plasma treatment using nitrousoxide gas (N₂O) decreases little after application of stress. Inparticular, the threshold voltage after application of stress decreasedby only 0.5 V from the threshold voltage before application of stress.

As discussed above, the decrease in threshold voltage after applicationof stress is suppressed when the oxide semiconductor layer 140 issubjected to a plasma treatment using nitrous oxide gas (N₂O). In otherwords, when an oxide semiconductor layer 140 that has a density ofstates satisfying formula (2) or formula (3) is included, the decreasein the threshold voltage of the thin film semiconductor device 100 issuppressed.

FIG. 11 is a graph showing the relationship between the power density ofthe plasma treatment and the threshold voltage according to thisembodiment.

As shown in FIG. 11, the change (|ΔV|) in threshold voltage is dependenton the power density of the plasma treatment with a minimum of about 0.5[W/cm²]. For example, the amount of change in threshold voltage issubstantially constant in the power density range of 0.5 [W/cm²] or moreand 1 [W/cm²] or less.

In the power density range of 0.2 [W/cm²] or more and 0.5 [W/cm²] orless, the amount of change in threshold voltage gradually increases withdecreasing power density. In contrast, in the power density range of 0.2[W/cm²] or less, the amount of change in threshold voltage rapidlyincreases with the decreasing power density.

Accordingly, the power density may be in the range of 0.2 [W/cm²] ormore in order to suppress the amount of change in threshold voltage.

In contrast, the standard deviation of the threshold voltage Vthgradually increases with the increasing power density. In the powerdensity range of 0.5 [W/cm²] or more, the standard deviation of thethreshold voltage increases rapidly.

Accordingly, the power density may be in the range of 0.5 [W/cm²] orless in order to suppress the standard deviation of the thresholdvoltage.

In view of the above, in order to suppress the negative shift of thethreshold voltage and manufacture a thin film semiconductor device 100having stable characteristics, the power density may be 1 [W/cm²] orless as shown in FIG. 10B. Similarly, in order to suppress the negativeshift of the threshold voltage and manufacture a thin film semiconductordevice 100 having stable characteristics, the power density may be 0.2[W/cm²] or more as shown in FIG. 11. Alternatively, as shown in FIG. 11,the power density may be 0.2 [W/cm²] or more and 0.5 [W/cm²] or less.

In other words, the decrease in the threshold voltage of the thin filmsemiconductor device 100 is suppressed when the thin film semiconductordevice 100 includes an oxide semiconductor layer 140 having a density ofstates of oxygen defects satisfying formula (2) or formula (3).Accordingly, the thin film semiconductor device 100 can exhibit morestable characteristics and higher reliability. Accordingly, when thethin film semiconductor device 100 is used as a drive transistor of adisplay for example, the luminance variation can be suppressed.

Effect of Suppressing Mobility Curve Peak

The effect of suppressing a mobility peak achieved by the thin filmsemiconductor device 100 of this embodiment will now be described withreference to FIG. 12.

FIG. 12 is a graph showing mobility-voltage characteristics of a thinfilm semiconductor device according to this embodiment.

The resistance of the back channel region of the oxide semiconductorlayer decreases due to oxygen defects occurred in the oxidesemiconductor layer. Since the density of states of oxygen defects inthe oxide semiconductor layer stays high if no plasma treatment usingnitrous oxide gas (N₂O) is performed, a mobility curve peak emerges.

In contrast, in the case where a plasma treatment is performed usingnitrous oxide gas (N₂O), the density of states of oxygen defects in theoxide semiconductor layer decreases and the decrease in resistance inthe back channel region is suppressed. Accordingly, as shown in FIG. 12,the mobility curve peak is suppressed by performing a plasma treatmentusing nitrous oxide gas (N₂O).

FIG. 13 is a graph showing the relationship between the pressurecondition of the plasma treatment and the mobility according to thisembodiment. In particular, FIG. 13 shows the mobility μ [cm²/Vs]exhibited when the oxide semiconductor layer 140 is subjected to aplasma treatment at a pressure of 0.3 [Torr] to 3.0 [Torr].

As shown in FIG. 13, the higher the pressure, the more suppressed is themobility curve peak. In other words, the mobility curve peak can befurther suppressed by performing a plasma treatment using nitrous oxidegas (N₂O) at a higher pressure. For example, the mobility curve peak canbe suppressed by performing a plasma treatment at a pressure of 2.0[Torr] or more. The plasma treatment may be performed at a pressure of3.0 [Torr] or more. Alternatively, the plasma treatment may be performedat a pressure of 1.5 [Torr] or more.

As discussed above, the mobility curve peak can be suppressed byperforming a plasma treatment at a pressure higher than a predeterminedpressure. Accordingly, a thin film semiconductor device having morestable characteristics can be manufactured by performing a plasmatreatment at a pressure higher than the predetermined pressure.

As discussed above, a thin film semiconductor device according to thisembodiment includes a substrate, a gate electrode formed above thesubstrate, an oxide semiconductor layer formed above the substrate so asto oppose the gate electrode, a first insulating layer formed on theoxide semiconductor layer, and a source electrode and a drain electrodeconnected to the oxide semiconductor layer, in which a density of statesDOS [eV⁻¹cm⁻³] of oxygen defects in the oxide semiconductor layersatisfies DOS≦1.710×10¹⁷×(Ec−E)²−6.468×10¹⁷×(Ec−E)+6.113×10¹⁷ providedthat 2.0 eV≦Ec−E≦2.7 eV where Ec [eV] is an energy level at a conductionband edge of the oxide semiconductor layer and E [eV] is a predeterminedenergy level of the oxide semiconductor layer.

Since the density of states of oxygen defects in the oxide semiconductorlayer is small, a negative shift in threshold voltage can be decreasedand the mobility curve peak can be decreased. Accordingly, a thin filmsemiconductor device having more stable characteristics can bemanufactured. In particular, a bottom-gate, channel-protective thin filmtransistor having more stable characteristics can be manufactured. Whenthe thin film transistor is used as a drive transistor of a display,luminance variation can be suppressed.

Second Embodiment

Next, a second embodiment is described. An organic EL display deviceaccording to this embodiment has the same structure as the organic ELdisplay device 10 of the first embodiment. Thus, the descriptiontherefor is omitted to avoid redundancy. A thin film semiconductordevice is described below.

Thin Film Semiconductor Device

A thin film semiconductor device according to this embodiment will nowbe described. A thin film semiconductor device according to thisembodiment is a bottom-gate, channel-etch thin film transistor.

FIG. 14 is a schematic cross-sectional view of a thin film semiconductordevice according to this embodiment.

As shown in FIG. 14, a thin film semiconductor device 200 according tothis embodiment includes a substrate 110, a gate electrode 120, a gateinsulating film 130, an oxide semiconductor layer 240, a channelprotective layer 250, a source electrode 260 s, and a drain electrode260 d. The oxide semiconductor layer 240 includes an oxygen rich layer241. Some structures that are substantially identical to those of thefirst embodiment are not described to avoid redundancy.

The oxide semiconductor layer 240 is formed above the substrate 110 soas to oppose the gate electrode 120. In particular, the oxidesemiconductor layer 240 is formed on the gate insulating film 130 so asto face the gate electrode 120. For example, an oxide semiconductorlayer 240 having an island form is formed on the gate insulating film130 and above the gate electrode 120. The same material used for formingthe oxide semiconductor layer 140 in the first embodiment can be used asthe material for the oxide semiconductor layer 240.

As shown in FIG. 14, the oxygen rich layer 241 is formed in a backchannel region of the oxide semiconductor layer 240. The oxygen richlayer 241 is a region of the oxide semiconductor layer 240 where oxygendefects are few. And the oxygen rich layer 241 is formed by performing aplasma treatment.

The density of states DOS [eV⁻¹cm⁻³] of oxygen defects in the oxidesemiconductor layer 240 satisfies, for example, formula (2).Alternatively, the density of states DOS [eV⁻¹cm⁻³] of oxygen defects inthe oxide semiconductor layer 240 may satisfy formula (3). As a result,as in the first embodiment, the density of states of oxygen defects inthe oxide semiconductor layer 240 is low. Accordingly, the negativeshift in threshold voltage and the mobility curve peak are decreased.Accordingly, the thin film semiconductor device 200 exhibits more stablecharacteristics.

The channel protective layer 250 is an example of a first insulatinglayer formed on the oxide semiconductor layer 240. For example, thechannel protective layer 250 is formed on the oxide semiconductor layer240, the source electrode 260 s, the drain electrode 260 d, and the gateinsulating film 130 so as to cover the oxide semiconductor layer 240,the source electrode 260 s, and the drain electrode 260 d. The materialfor the channel protective layer 250 may be, for example, the same asthat of the oxide semiconductor layer 140 of the first embodiment.

The source electrode 260 s and the drain electrode 260 d are inpredetermined forms and formed on the oxide semiconductor layer 240. Inparticular, the source electrode 260 s and the drain electrode 260 d arearranged to oppose each other with a space therebetween in the directionhorizontal to the substrate on the oxide semiconductor layer 240 (oxygenrich layer 241). As shown in FIG. 14, the source electrode 260 s and thedrain electrode 260 d are formed on the oxide semiconductor layer 240and the gate insulating film 130 so as to cover the end surfaces of theoxide semiconductor layer 240 in the direction horizontal to thesubstrate. For example, the same material as the gate electrode 120 canbe used as the material for the source electrode 260 s and the drainelectrode 260 d.

Method for Manufacturing Thin Film Semiconductor Device

A method for manufacturing a thin film semiconductor device according tothis embodiment will now be described with reference to FIGS. 15A to15H. FIGS. 15A to 15H are schematic cross-sectional views illustrating amethod for manufacturing a thin film semiconductor device according tothis embodiment.

Formation of the gate electrode 120 shown in FIG. 15A and formation ofthe gate insulating film 130 shown in FIG. 15B are the same as those ofthe first embodiment and thus the description therefor is omitted (referto FIGS. 7A and 7B). Formation of an oxide semiconductor film 240 ashown in FIG. 15C, formation of the oxide semiconductor layer 240 shownin FIG. 15D, and a plasma treatment shown in FIG. 15E are alsosubstantially the same as those of the first embodiment and thedescription therefor is omitted (refer to FIGS. 7C to 7E). Here, theoxide semiconductor film 240 a, the oxide semiconductor layer 240, andthe oxygen rich layer 241 respectively correspond to the oxidesemiconductor film 140 a, the oxide semiconductor layer 140, and theoxygen rich layer 141.

Next, as shown in FIG. 15F, a source electrode 260 s and a drainelectrode 260 d are formed on the oxide semiconductor layer 240 (oxygenrich layer 241). For example, a source electrode 260 s and a drainelectrode 260 d are formed on the oxide semiconductor layer 240 and thegate insulating film 130 so as to cover the end surfaces of the oxidesemiconductor layer 240 and to be spaced from each other in thedirection horizontal to the substrate.

In particular, a Mo film, a Cu film, and a CuMn film are sequentiallyformed on the oxide semiconductor layer 240 and the gate insulating film130 by a sputtering method. The Mo film, the Cu film, and the CuMn filmare patterned by a photolithography method and a wet etching method soas to form a source electrode 260 s and a drain electrode 260 d.

The thickness of the source electrode 260 s and the drain electrode 260d is, for example, 100 nm to 500 nm. The Mo film, the Cu film, and theCuMn film can be wet-etched by using a chemical solution containingaqueous hydrogen peroxide (H₂O₂) and an organic acid.

At this stage, a part of the upper surface of the oxide semiconductorlayer 240 (oxygen rich layer 241) is sometimes removed by etching.Oxygen defects occur in the oxide semiconductor layer 240 as a result ofsuch etching.

Next, as shown in FIG. 15G, a plasma treatment is performed by usingoxygen-containing gas. Because of this plasma treatment, oxygen defectscaused by the damage inflicted by etching in forming the sourceelectrode 260 s and the drain electrode 260 d can be decreased.

In particular, the oxide semiconductor layer 240 is subjected to aplasma treatment using nitrous oxide gas (N₂O) as the introduced gas ata power density of 0.2 [W/cm²] or more and 1 [W/cm²] or less and at apressure of 2.0 [Torr] or more. The time for the plasma treatment is,for example, 30 seconds to 5 minutes. The conditions for performing aplasma treatment are not limited to these. For example, the introducedgas may be oxygen gas (O₂), ozone gas (O₃), nitrogen dioxide gas (NO₂),or the like.

Next, as shown in FIG. 15H, a channel protective layer 250 is formed onthe oxide semiconductor layer 240, the source electrode 260 s, and thedrain electrode 260 d. For example, a channel protective layer 250 isformed on the gate insulating film 130 so as to cover the oxidesemiconductor layer 240 (oxygen rich layer 241), the source electrode260 s, and the drain electrode 260 d.

In particular, a channel protective layer 250 can be formed by forming asilicon oxide film on the gate insulating film 130 by a plasma CVDmethod. The thickness of the channel protective layer 250 is, forexample, 50 to 500 nm.

As a result of performing the above-described processes, a thin filmsemiconductor device 200 is manufactured.

As in the first embodiment, the density of states of oxygen defects inthe oxide semiconductor layer can be sufficiently decreased byperforming a plasma treatment using oxygen-containing gas. Since thedensity of states of oxygen defects in the oxide semiconductor layer issmall, the negative shift in threshold voltage can be decreased and themobility curve peak can be decreased. Accordingly, a bottom-gate,channel-etch thin film transistor having more stable characteristics canbe manufactured. When the thin film transistor is used as a drivetransistor of a display, luminance variation can be suppressed.

Third Embodiment

Next, a third embodiment is described. An organic EL display deviceaccording to this embodiment has the same structure as the organic ELdisplay device 10 of the first embodiment. Thus, the descriptiontherefor is omitted to avoid redundancy. A thin film semiconductordevice is described below.

Thin Film Semiconductor Device

A thin film semiconductor device of this embodiment will now bedescribed. A thin film semiconductor device according to this embodimentis a top-gate thin film transistor.

FIG. 16 is a schematic cross-sectional view of a thin film semiconductordevice according to this embodiment.

As shown in FIG. 16, a thin film semiconductor device 300 according tothis embodiment includes a substrate 110, a gate electrode 320, a gateinsulating film 330, an oxide semiconductor layer 340, an insulatinglayer 350, a source electrode 360 s, and a drain electrode 360 d. Theoxide semiconductor layer 340 includes an oxygen rich layer 341. Somestructures that are substantially identical to those of the firstembodiment are not described to avoid redundancy.

A gate electrode 320 having a predetermined form is formed above thesubstrate 110. In particular, the gate electrode 320 is formed on thegate insulating film 330 at a position opposing the oxide semiconductorlayer 340. The material for the gate electrode 320 may be the samematerial as that for the gate electrode 120 of the first embodiment.

The gate insulating film 330 is an example of a first insulating filmformed on the oxide semiconductor layer 340. For example, a gateinsulating film 330 is formed on the oxide semiconductor layer 340 andthe substrate 110 so as to cover the oxide semiconductor layer 340. Inother words, the gate electrode 320 is formed above the oxidesemiconductor layer 340 so as to oppose the oxide semiconductor layer340. The material for the gate insulating film 330 is, for example, thesame as that for the gate insulating film 130 in the first embodiment.

Holes are formed in part of the gate insulating film 330 so as topenetrate through the gate insulating film 330. In other words, contactholes for exposing part of the oxide semiconductor layer 340 (oxygenrich layer 341) are formed in the gate insulating film 330.

The oxide semiconductor layer 340 is formed on the substrate 110. Inparticular, an oxide semiconductor layer 340 having an island form isformed on the substrate 110. The material for the oxide semiconductorlayer 340 is, for example, the same as that for the oxide semiconductorlayer 140 in the first embodiment.

As shown in FIG. 16, the oxygen rich layer 341 is formed in thefront-channel-side region of the oxide semiconductor layer 340(hereinafter this region is referred to as the front channel region).The oxygen rich layer 341 is a region having fewer oxygen defects amongthe oxide semiconductor layer 340 and is formed by a plasma treatment.The front channel region is defined as a region in the oxidesemiconductor layer that opposes the gate electrode (the region on theopposite side of the back channel region).

The density of states DOS [eV⁻¹cm⁻³] of oxygen defects in the oxidesemiconductor layer 340 satisfies, for example, formula (2).Alternatively, the density of states DOS [eV⁻¹cm⁻³] of oxygen defects inthe oxide semiconductor layer 340 may satisfy formula (3). As a result,as in the first embodiment, the density of states of oxygen defects inthe oxide semiconductor layer 340 is decreased and the negative shift inthreshold voltage and the mobility curve peak are decreased.Accordingly, the thin film semiconductor device 300 exhibits more stablecharacteristics.

The insulating layer 350 is formed on the gate electrode 320 and thegate insulating film 330. For example, the insulating layer 350 isformed on the gate electrode 320 and the gate insulating film 330 so asto cover the gate electrode 320. The material for the insulating layer350 is, for example, the same as that for the channel protective layer150 in the first embodiment.

Holes that penetrate through the insulating layer 350 are formed in apart of the insulating layer 350. In other words, contact holes areformed in the insulating layer 350. The oxide semiconductor layer 340(oxygen rich layer 341) is connected to the source electrode 360 s andthe drain electrode 360 d through the contact holes formed in theinsulating layer 350 and the contact holes formed in the gate insulatingfilm 330.

The source electrode 360 s and the drain electrode 360 d are inpredetermined forms and are formed on the insulating layer 350. Inparticular, the source electrode 360 s and the drain electrode 360 d areconnected to the oxide semiconductor layer 340 (oxygen rich layer 341)through contact holes formed in the insulating layer 350 and the gateinsulating film 330. The source electrode 360 s and the drain electrode360 d are arranged to oppose each other with a space therebetween in thedirection horizontal to the substrate on the insulating layer 350. Thematerial for the source electrode 360 s and the drain electrode 360 dmay be, for example, the same as the material for the gate electrode320.

Method for Manufacturing Thin Film Semiconductor Device

A method for manufacturing a thin film semiconductor device according tothis embodiment will now be described with reference to FIGS. 17A to17G. FIGS. 17A to 17G are schematic cross-sectional views illustrating amethod for manufacturing a thin film semiconductor device according tothis embodiment.

First, as shown in FIG. 17A, a substrate 110 is prepared and an oxidesemiconductor film 340 a is formed on the substrate 110. For example, anoxide semiconductor film 340 a is formed on the substrate 110 by asputtering method. The thickness of the oxide semiconductor film 340 ais, for example, the same as that of the oxide semiconductor film 140 ain the first embodiment.

Next, as shown in FIG. 17B, an oxide semiconductor layer 340 in apredetermined form is formed on the substrate 110. For example, an oxidesemiconductor layer 340 is formed by patterning the oxide semiconductorfilm 340 a. The specific patterning method may be the same as that usedin forming the oxide semiconductor layer 140, for example.

Next, as shown in FIG. 17C, a plasma treatment is performed by usingoxygen-containing gas. For example, the oxide semiconductor layer 340 isexposed to a plasma 170 to form an oxygen rich layer 341 in the oxidesemiconductor layer 340. The thickness of the oxygen rich layer 341 is,for example, 5 to 50 nm. The plasma treatment conditions such asintroduced gas, power density, pressure, and the like, are, for example,the same as those of the first embodiment.

As a result of performing a plasma treatment on the oxide semiconductorlayer 340, an oxygen rich layer 341 is formed in the region exposed tothe plasma. As shown in FIG. 17C, the oxygen rich layer 341 is formed inthe front channel region of the oxide semiconductor layer 340.

Next, as shown in FIG. 17D, a gate insulating film 330 is formed on theoxide semiconductor layer 340. For example, a gate insulating film 330is formed on the oxide semiconductor layer 340 and the substrate 110 bya plasma CVD method so as to cover the oxide semiconductor layer 340(oxygen rich layer 341). The thickness of the gate insulating film 330is, for example, the same as that of the gate insulating film 130 in thefirst embodiment.

Next, as shown in FIG. 17E, a gate electrode 320 is formed above thesubstrate 110. For example, a metal film is formed on the gateinsulating film 330 by a sputtering method and then processed through aphotolithography method and a wet etching method so as to form a gateelectrode 320 having a predetermined form. The gate electrode 320 isformed at a position that opposes the oxide semiconductor layer 340. Thethickness of the gate electrode 320 is, for example, the same as that ofthe gate electrode 120 in the first embodiment.

Next, as shown in FIG. 17F, an insulating layer 350 is formed on thegate electrode 320. For example, an insulating layer 350 is formed onthe gate electrode 320 and the gate insulating film 330 so as to coverthe gate electrode 320. The thickness of the insulating layer 350 is,for example, the same as that of the channel protective layer 150 in thefirst embodiment.

The insulating layer 350 and the gate insulating film 330 are patternedso as to have predetermined forms. In particular, contact holes areformed in the insulating layer 350 and the gate insulating film 330 soas to expose part of the oxide semiconductor layer 340 (oxygen richlayer 341). For example, contact holes are formed by etching away partof the insulating layer 350 and the gate insulating film 330.

To be specific, first, part of the insulating layer 350 and the gateinsulating film 330 are etched by a photolithography method and a dryetching method. As a result of this etching, contact holes are formed inregions that will form a source contact region and a drain contactregion of the oxide semiconductor layer 340. For example, in the casewhere the insulating layer 350 and the gate insulating film 330 aresilicon oxide films, a reactive ion etching (RIE) method may be employedas a dry etching method. The etching gas that can be used in dry etchingis, for example, carbon tetrafluoride (CF₄) and oxygen gas (O₂). Theparameters such as gas flow rate, pressure, applied power, and frequencyare appropriately set depending on the substrate size, thickness of thefilm to be etched, etc.

Next, as shown in FIG. 17G, a source electrode 360 s and a drainelectrode 360 d connected to the oxide semiconductor layer 340 areformed. For example, a source electrode 360 s and a drain electrode 360d having predetermined forms are formed on the insulating layer 350while filling the contact holes formed in the insulating layer 350 andthe gate insulating film 330.

In particular, a source electrode 360 s and a drain electrode 360 d areformed on the insulating layer 350 and in the contact holes so as bespaced from each other. To be more specific, a Mo film, a Cu film, and aCuMn film are sequentially formed on the insulating layer 350 and in thecontact holes by a sputtering method. The Mo film, the Cu film, and theCuMn film are then patterned by a photolithography method and a wetetching method so as to form a source electrode 360 s and a drainelectrode 360 d. The thickness of the source electrode 360 s and thedrain electrode 360 d is, for example, the same as that of the sourceelectrode 160 s and the drain electrode 160 d in the first embodiment.

As a result of performing the above-described processes, a thin filmsemiconductor device 300 is manufactured.

As in the first embodiment, the density of states of oxygen defects inthe oxide semiconductor layer is low due to a plasma treatment usingoxygen-containing gas. Accordingly, the negative shift in thresholdvoltage can be decreased and the mobility curve peak can be decreased.Thus, a top-gate thin film transistor having more stable characteristicscan be manufactured. When the thin film transistor is used as a drivetransistor of a display, luminance variation can be suppressed.

Other Embodiments

The first to third embodiments have been described as illustrativeexamples of the technology disclosed in this application. However, thetechnology in this disclosure is not limited to these embodiments andcan be applied to other embodiments which involve appropriatealterations, substitutions, additions, omissions, and the like.

For example, in the embodiments above, a plasma treatment that usesnitrous oxide gas (N₂O) as the introduced gas and conditions thereof aredescribed. However, the treatment and conditions are not limited tothese. Different pressure conditions and power density conditions may beemployed in the plasma treatment depending on the gas used in the plasmatreatment. For example, the conditions that form an oxide semiconductorlayer with a density of states of oxygen defects satisfying formula (2)or formula (3) may be appropriately selected depending on the type ofgas used in the treatment.

In the first embodiment, contact holes for the source electrode 160 sand the drain electrode 160 d are formed in the channel protective layer150 after an insulating film was formed on the entire surface as shownin FIGS. 7F and 7G. However, formation of the contact holes is notlimited to this. For example, a channel protective layer 150 that haspreviously been patterned into a predetermined form so as to expose theoxide semiconductor layer 140 may be formed.

In other words, in the step of forming a channel protective layer 150,it is sufficient if a channel protective layer 150 is formed so as toexpose a part of the oxide semiconductor layer 140. Moreover, in thestep of forming a source electrode 160 s and a drain electrode 160 d, itis sufficient if a source electrode 160 s and a drain electrode 160 dare formed so as to connect to the oxide semiconductor layer 140 throughthe exposed portion.

The same applies to formation of the layers, such as the oxidesemiconductor layer 140, that need to be patterned into predeterminedforms. That is, instead of performing patterning after forming a film onthe entire surface, an oxide semiconductor layer 140 that has beenpreviously patterned into a predetermined form may be formed. Thisapplies to other embodiments also.

In the above-discussed embodiments, the oxide semiconductor used to formthe oxide semiconductor layer is not limited to amorphous IGZO. Forexample, a polycrystalline semiconductor such as polycrystalline InGaOcan be used.

In the above-discussed embodiments, an organic EL display device isdescribed as a display device that includes a thin film semiconductordevice. However, the thin film semiconductor devices described in theseembodiments can also be applied to other types of display devices thatuse active matrix substrates, such as liquid crystal display devices.

The display devices (display panels) such as the organic EL displaydevices described above can be used as flat panel displays. The displaydevices can also be applied to all types of electronic appliances havingdisplay panels, such as television sets, personal computers, andcellular phones. In particular, the display devices are suitable for usein large-screen, high-definition display devices.

The present invention encompasses individual embodiments andmodifications with a variety of alterations conceivable by personsskilled in the art. The present invention also encompasses embodimentsrealized by freely combining the structural elements and functions inthe embodiments and modifications.

A thin film semiconductor device and a method for manufacturing the thinfilm semiconductor device according to this disclosure can be used in,for example, a display device such as an organic EL display device.

What is claimed is:
 1. A thin film semiconductor device comprising: asubstrate; a gate electrode disposed above the substrate; an oxidesemiconductor layer disposed above the substrate so as to oppose thegate electrode; a first insulating layer disposed on the oxidesemiconductor layer; and a source electrode and a drain electrode eachconnected to the oxide semiconductor layer, wherein a density of statesDOS [eV⁻¹cm⁻²] of oxygen defects in the oxide semiconductor layersatisfies the following relationship:DOS≦1.710×10¹⁷×(Ec−E)²−6.468×10¹⁷×(Ec−E)+6.113×10¹⁷ provided that 2.0eV≦Ec−E≦2.7 eV where Ec [eV] is an energy level of a conduction bandedge of the oxide semiconductor layer and E [eV] is a predeterminedenergy level of the oxide semiconductor layer.
 2. The thin filmsemiconductor device according to claim 1, wherein the density of statesDOS [eV⁻¹cm⁻³] satisfies the following relationship:DOS≦1.332×10¹⁰×(Ec−E)^(14.65) provided that 2.0 eV≦Ec−E≦2.7 eV.
 3. Thethin film semiconductor device according to claim 1, further comprising:a second insulating layer disposed on the gate electrode, wherein theoxide semiconductor layer is disposed on the second insulating layer, acontact hole for exposing part of the oxide semiconductor layer isdisposed in the first insulating layer, and the source electrode and thedrain electrode are disposed on the first insulating layer and connectedto the oxide semiconductor layer through the contact hole.
 4. The thinfilm semiconductor device according to claim 1, further comprising: asecond insulating layer disposed on the gate electrode, wherein theoxide semiconductor layer is disposed on the second insulating layer,the source electrode and the drain electrode are disposed on the oxidesemiconductor layer, and the first insulating layer is disposed on thesource electrode, the drain electrode, and the oxide semiconductorlayer.
 5. The thin film semiconductor device according to claim 1,wherein the gate electrode is disposed on the first insulating layer. 6.The thin film semiconductor device according to claim 1, wherein theoxide semiconductor layer comprises a transparent amorphous oxidesemiconductor.
 7. A method for manufacturing a thin film semiconductordevice, the method comprising steps of: (a) forming a gate electrodeabove a substrate; (b) forming an oxide semiconductor layer at aposition above the substrate and opposing the gate electrode; (c)performing a plasma treatment on the oxide semiconductor layer by usinggas containing oxygen; (d) forming a first insulating layer on the oxidesemiconductor layer; and (e) forming a source electrode and a drainelectrode each connected to the oxide semiconductor layer, wherein, inthe performing step (c) the plasma treatment is performed in particularconditions that a density of states DOS [eV⁻¹cm⁻³] of oxygen defects inthe oxide semiconductor layer satisfies the following relationship:DOS≦1.710×10¹⁷×(Ec−E)²−6.468×10¹⁷×(Ec−E)+6.113×10¹⁷ provided that 2.0eV≦Ec−E≦2.7 eV, where Ec [eV] is an energy level of a conduction bandedge of the oxide semiconductor layer and E [eV] is a predeterminedenergy level of the oxide semiconductor layer.
 8. The method formanufacturing a thin film semiconductor device according to claim 7,wherein the gas contains nitrous oxide, and a power density of theplasma treatment is 1 [W/cm²] or less.
 9. The method for manufacturing athin film semiconductor device according to claim 8, wherein the powerdensity of the plasma treatment is 0.2 [W/cm²] or more.
 10. The methodfor manufacturing a thin film semiconductor device according to claim 7,wherein the gas contains nitrous oxide, and in the performing step (c)the plasma treatment is performed at a pressure of 2.0 [Torr] or more.11. The method for manufacturing a thin film semiconductor deviceaccording to claim 7, further comprising a step of: (f) forming a secondinsulating layer on the gate electrode, wherein, in the forming step (b)the oxide semiconductor layer is formed on the second insulating layer,in the forming step (d) the first insulating layer is formed so thatpart of the oxide semiconductor layer is exposed, and in the formingstep (e) the source electrode and the drain electrode are formed so asto connect to the oxide semiconductor layer at the exposed part.
 12. Themethod for manufacturing a thin film semiconductor device according toclaim 7, further comprising a step of: (f) forming a second insulatinglayer on the gate electrode, wherein in the forming step (b) the oxidesemiconductor layer is formed on the second insulating layer, in theforming step (e) the source electrode and the drain electrode are formedon the oxide semiconductor layer, and in the forming step (d) the firstinsulating layer is formed on the source electrode, the drain electrode,and the oxide semiconductor layer.
 13. The method for manufacturing athin film semiconductor device according to claim 12, wherein theperforming step (c) is performed before the forming step (e).
 14. Themethod for manufacturing a thin film semiconductor device according toclaim 7, wherein in the forming step (a) the gate electrode is formed onthe first insulating layer.
 15. The method for manufacturing a thin filmsemiconductor device according to claim 7, wherein the performing step(c) is performed before the forming step (d).
 16. The method formanufacturing a thin film semiconductor device according to claim 7,wherein the oxide semiconductor layer comprises a transparent amorphousoxide semiconductor.